17 February 2014

Life cycle impact benchmarking between LIB packs with SiNW and graphite anode. Units of the X-axis values are different and shown under each impact category name on Y-axis. Credit: ACS, Li et al. Click to enlarge.

A lifecycle assessment (LCA) of silicon nanowire (SiNW) anodes for Li-ion batteries (LIBs) by researchers at the University of Wisconsin-Milwaukee has concluded that a LIB pack using SiNW anodes from metal-assisted chemical etching could have environmental impacts comparable with those of a conventional Li-ion battery pack, while significantly increasing the battery energy storage. The study is published in the ACS journal Environmental Science & Technology.

The LCA was based on the average US driving and electricity supply conditions. The researchers characterized nanowastes and nanoparticle emissions from the SiNW synthesis. The results showed that more than 50% of most characterized impacts are generated from the battery operations, while the battery anode with SiNW material contributes to around 15% of global warming potential and 10% of human toxicity potential.

Recently it has been found that reducing the silicon size below 150 nm can effectively address the volume expansion issue and obtain high-capacity LIBs. As a result, high-capacity LIB technologies are currently under development at lab scale with silicon nanostructured anode materials. In particular, silicon nanowires (SiNW) are widely studied as a promising anode material for high-capacity LIBs due to its low cost of fabrication and volume production potential. Although using silicon nanomaterials on LIB is technically beneficial, the associated environmental impacts might be of concern because nanoscale manufacturing is usually energy intensive, relying heavily on toxic chemicals for manufacturing operations, and generating a large proportion of wastes and emissions including nanowastes and nanoparticle emissions from the nanomanufacturing process.

… Review of the literature indicates SiNW for LIB anode are mainly prepared in two ways: chemical vapor deposition (CVD) or wet chemistry method. When compared, wet chemistry method is better than CVD in terms of both economic and environmental performance. In particular, a recently developed wet chemistry technique using metal-assisted chemical etching of silicon powder to produce nanowire structure, is easily scalable and cost-effective.

… Although silicon nanomaterials are considered the ideal anode material for high-capacity LIBs, until now there has been no study conducted on the potential environmental impacts of LIBs with silicon nanomaterial for future EV applications.

For the study, the UW-Milwaukee team selected the metal-assisted chemical etching method for the SiNW synthesis due to its simplicity, low cost, and scalable advantages. Although future industrial-scale productions will be different from current lab-scales, the team noted, the fundamental mechanism and processes should be similar.

For the LCA, considering the high capacity of each LIB cell with SiNW anode, the researchers configured the battery pack as a 43.2 kWh battery system comprising 12 modules with each module containing 12 LIB prismatic cells. The battery pack had a total weight of 120 kg. Each LIB cell had a 3.65-V voltage and 27-Ah capacity.

The 43.2 kWh battery pack was used in an average mid-sized EV with a weight of 4270 lb (1936.8 kg), and an average driving distance of 200,000 km (124,000 miles) during a 10-year service life. A single battery pack is assumed to power the vehicle during its whole life. The driving mix of the EV was 55% urban and 45% highway. The average US electricity mix was supposed for the battery charging and operations.

A conventional battery pack using graphite anode with the same capacity was the a basis for comparison of the life cycle impact results. The conventional battery pack had 36 modules; each module had 12 cells. The conventional battery pack had a total weight of 360 kg.

The study showed that most of the SiNW battery life cycle impacts were generated in the battery use and material production stages.

The largest impact from the battery use stage is mainly from the primary energy consumption (2.94 × 105 MJ) during the 10-year service life of the EV. The total primary energy consumption in the use stage is about 18 times that of the embedded energy in the battery pack.

The SiNW anode as produced with large amount of embedded energy and toxic chemicals, contributes to 15% of GWP, 18% of ADP, 17% of POP, and 10% of HTP, respectively, to each corresponding life cycle impact category of the battery pack.

The results demonstrate that the major opportunity for reducing the life cycle impacts of the battery pack is to use clean energy supply for battery operation, such as solar and wind electricity, which could reduce these environmental impacts significantly.

—Li et al.

Comparing the results of the SiNW anode battery pack to the conventional graphite pack, they found that the life cycle impacts from individual life cycle stages of the two battery packs are quite different. For example, the life cycle impacts of the conventional battery pack using carbon graphite are dominated by the battery use phase which contributes to 78% of ADP, 78% of GWP, 64% of AP, 75% of EP, 80% of POP, and 83% of HTP. Whereas for the battery pack using SiNW anode, the contributions of battery use stage are much lower because of the increased impacts from the battery production.

Overall the differences of the impacts between the two battery packs, in average, are moderate. Considering the uncertainties in the lab-scale inventory data and the potential of impact reduction in future industrial scale productions, we can say the life cycle environmental impacts of the two battery packs are comparable with each other. This is encouraging since some nanoscale manufacturing technologies could generate impacts several orders of magnitude higher than those of conventional manufacturing technologies. This study reveals that appropriate nanomanufacturing technologies could be employed to enhance the technical performance of manufactured products while maintaining the same level of environmental impacts.